1. Field of Invention
The present invention relates to a method of producing a container for holding a columnar member in a cylindrical housing, with a shock absorbent member wrapped around the columnar member, and more particularly a method of producing a catalytic converter for holding a catalyst substrate with a shock absorbent mat wrapped around it in a cylindrical housing, and relates to an apparatus of producing the same.
2. Description of Related Arts
A container for holding a columnar member having a honeycomb structure and functioning as a fluid filter in a metallic cylindrical housing through a shock absorbent member has been used for a fluid treatment device, and provided for purifying various fluids. In an exhaust system of an automotive vehicle, for example, a catalytic converter, a diesel particulate filter (abbreviated as DPF) and the like have been used, and equipped with a fragile ceramic columnar member of a honeycomb structure, for a catalyst substrate, filter or the like (hereinafter, referred to as catalyst substrate). The honeycomb columnar member is held in the metallic cylindrical housing thorough the shock absorbent member such as a ceramic mat or the like, to constitute the fluid treatment device, an example of which is the catalytic converter. In order to produce the container for holding the columnar member such as the catalytic converter, generally employed is such a method for wrapping the shock absorbent member around the catalyst substrate, and stuffing them into the cylindrical housing, with the shock absorbent member being compressed.
For example, Japanese Patent Laid-open Publication No. 2001-355438 proposes a method of producing a catalytic converter, by measuring the outer diameter of a catalyst substrate, when the catalyst substrate with a holding material mounted around its periphery is stuffed (pressed) into a holding cylinder, and then stuffing the catalyst substrate with the holding material mounted thereon into the holding cylinder with its inner diameter adapted for the measured outer diameter. Also, it is proposed to measure the outer diameter of the holding material mounted on the catalyst substrate, and stuff the catalyst substrate with the holding material mounted thereon into the holding cylinder with its inner diameter adapted for the measured outer diameter. Furthermore, it is proposed to measure the outer diameter of the holding material in such a state that a certain pressure is applied to the holding material. It is also proposed to select a holding cylinder having a proper inner diameter, out of a plurality of holding cylinders with various inner diameters different from one another, which were provided in advance.
In contrast, it is proposed such a method called as xe2x80x9csizingxe2x80x9d or xe2x80x9ccalibratingxe2x80x9d, wherein after the catalyst substrate and a shock absorbent mat mounted thereon were inserted into a cylindrical member, the diameter of the cylindrical member is reduced until the shock absorbent mat will be compressed to the most appropriate compressed amount, as disclosed in Japanese Patent Laid-open Publication Nos. 64-60711, 8-42333, 9-170424, 9-234377, U.S. Pat. Nos. 5,329,698, 5,755,025, 6,389,693, and European Patent Publication No. EP0982480A2 and so on. Among them, in Japanese Patent Laid-open Publication No. 9-234377, it is proposed to reduce a casing along its entire longitudinal length, in order to solve a problem in its prior art as disclosed in Japanese Patent Laid-open Publication No. 2-268834. In the former Publication, it is stated about the latter Publication that there is disclosed a catalytic converter with a central portion of a tubular body reduced in diameter to form a compressed portion, and compress a support mat to support a ceramic honeycomb body in the casing. And, it is stated in the former Publication that the above problem will be caused, as a clearance between the outer circumference of the honeycomb body and the inner circumference of the casing is large in a direction from an end of the compressed portion toward cone portions which are not reduced in diameter.
In the U.S. Pat. No. 5,755,025, a process for manufacturing catalytic converters is proposed, as described in its abstract, by pushing monoliths and a surrounding support jacket into prefabricated tubes, whose cross section essentially corresponds to the profile of the monolith plus an addition for the support jacket. In the abstract, it is described that the dimensions of the tube (housing) are adapted to a constant gap(s) from the monolith by sizing (calibrating) the prefabricated tubes which initially have a smaller cross section. Thus, substantially the same process as stuffing the substrate and mat into the housing as described before has been employed. Furthermore, in the U.S. Pat. No. 6,389,693, a method of manufacturing a catalytic converter is proposed, by resizing a container over substantially the entire portion of its length which is occupied by the wrapped substrate to a predetermined metal shell/container outside diameter (OD). The predetermined outside diameter is characterized by the equation OD=D+2T1+2T2, wherein xe2x80x9cDxe2x80x9d is a diameter measure of the substrate, xe2x80x9cT1xe2x80x9d is the supporting mat target thickness and xe2x80x9cT2xe2x80x9d is a container wall thickness measure. Likewise, in the European Patent Publication No. EP0982480A2, it is described in its abstract that the subsequent to loading a mat and substrate into a can, the measurement of the substrate is used to direct the degree to which the can is reduced in outside dimension such that a selected annulus is created between the substrate and can, the annulus being occupied by the mat.
With respect to Japanese Patent Laid-open Publication No. 2000-45762 cited in Japanese Patent Laid-open Publication No. 2001-355438, a method for reducing a cylindrical member by a spinning process. Furthermore, there is disclosed in Japanese Patent Laid-open Publication No. 2001-107725, a method for producing a catalytic converter by reducing a diameter of a cylindrical member with a shock absorbent member held therein to hold a substrate catalyst, according to a spinning process using a plurality of spinning rollers revolved about the cylindrical member. As for a necking process applied to an end portion of the cylindrical member, an offset spinning process is disclosed in Japanese Patent No. 2957153, and an oblique spinning process is disclosed in Japanese Patent No. 2957154. And, a spinning apparatus is disclosed in Japanese Patent Laid-open Publication No. 2001-137962.
In the Japanese Patent Laid-open Publication No. 2001-355438 as described above, it is described that it is preferable to measure the outer diameter of the holding material, in such a state that the holding material 3 is applied with the same pressure (holding pressure) as the pressure which will be applied to the holding material 3 when the catalyst substrate 2 is stuffed into (pressed into) the holding cylinder 1. According to the method as described above, however, it is impossible to estimate the pressure which will be applied to the holding material in the later process, and no explanation about this matter has been described. Therefore, the above description that the holding material 3 is applied with the same pressure as the pressure which will be applied to the holding material 3 when the catalyst substrate 2 is pressed into the holding cylinder 1, is merely a desire or hope, and nothing is disclosed to show that it will be possibly realized. In addition, it is described that as for a base member of the holding cylinder 1, used is the one having its inner diameter which will enable to have the stuffed holding material 3 apply the appropriate pressure to the catalyst substrate 2. It is also stated that it can be achieved to select the one having the appropriate inner diameter, out of a plurality of base members having different inner diameters from one another prepared in advance. Therefore, it is apparent that the holding cylinder 1 is not the one having its inner diameter to be adjusted in accordance with the result of the measurement of the outer diameter of the holding material 3, in the state that the holding material 3 is applied with the same pressure as the pressure which will be applied to the holding material 3 when stuffed into the holding cylinder 1, which measurement can not be made in fact, as described above. After all, it is not clear in the Japanese Patent Laid-open Publication No. 2001-355438, how the outer diameter of the holding material 3 is measured, in what state the pressure applied to it, nor how and what type of the measured result is used.
On the contrary, according to the conventional method by the stuffing process, on the basis of density of a shock absorbent mat served as the shock absorbent member, which is called as GBD (abbreviation of gap bulk density), an annular clearance between the outer diameter of the catalyst substrate and the inner diameter of the cylindrical housing is determined, in general. The GBD is the value obtained from [weight per unit area/bulk gap]. According to the bulk density of the shock absorbent mat, pressure (Pascal) is created to hold the catalyst substrate. The pressure has to be adjusted to a value which will not exceed the strength of the catalyst substrate, and to a value which is capable of holding the catalyst substrate applied with vibration and exhaust gas pressure not to be moved in the cylindrical housing. Therefore, the shock absorbent member (shock absorbent mat) is required to be stuffed to create the GBD within a predetermined design range, and the GBD is required to be maintained for a life cycle of the product.
According to the conventional method by the stuffing process as described above, however, an error in the outer diameter of the catalyst substrate necessarily caused when producing it, an error in the inner diameter of the cylindrical housing, and an error in weight per unit area of the shock absorbent mat disposed between them are added to create an error in GBD. Therefore, it can not be a practical solution for mass-production to find a combination of each member adapted to minimize the error in GBD. Furthermore, the GBD itself is varied depending upon the property or individual difference of the shock absorbent mat. And, the GBD relies on the value measured on a flat plane, so that it does not indicate the value measured in the case where the shock absorbent mat is tightly wrapped around the catalyst substrate. Accordingly, it has been desired to stuff the catalyst substrate properly into the cylindrical housing, without relying on the GBD.
On the contrary, according to the conventional sizing method, it is proposed to measure the outer diameter of the catalyst substrate and the inner diameter of the cylindrical housing in advance, to determine an appropriate compression amount for the shock absorbent member, and then reduce the diameter by the determined compression amount. However, it is difficult to determine whether the final compression amount is appropriate or not, because the errors of each catalyst substrate and each shock absorbent member are added, and the thickness of each shock absorbent member wrapped around each catalyst substrate is varied. In addition, the difficulty results from the fact that when reducing the diameter of the metallic cylindrical member, it is required to reduce the diameter slightly smaller than a target diameter (so called overshooting), in view of a spring back of the cylindrical member. As a result, excessive compression force might be created. Also, the difficulty is resulted from the fact that when reducing the diameter of the metallic cylindrical member, unavoidable change in thickness of its wall is caused, i.e., the wall thickness is increased when reducing the diameter. Consequently, it is so difficult to determine a true inner diameter (position of inner wall surface), i.e., accurate reducing amount, that the mass-production can not be realized.
In order to solve the problem caused by the overshooting or the like as described above, such a method for measuring the outer diameter of the catalyst substrate in advance, and reducing the diameter of the housing on the basis of the compression amount or target thickness of the shock absorbent mat has been proposed, in the U.S. Pat. Nos. 5,755,025, 6,389,693 and European Patent Publication No. EP0982480A2 as cited before. However, nothing is considered about the various errors caused with respect to the shock absorbent mat including the error in weight per unit area of the shock absorbent mat as described before. Therefore, the ultimate problem about the error in pressure applied to the catalyst substrate can not be avoided, as will be explained in detail hereinafter. At the outset, with respect to a holding force for holding the catalyst substrate in a predetermined position within the cylindrical housing, the holding force in a radial direction of the cylindrical housing corresponds to the pressure reproduction force of the shock absorbent member acting on the outer surface of the catalyst substrate and the inner surface of the cylindrical housing, in a direction perpendicular to those surfaces. On the other hand, with respect to the cylindrical housing fixed to the exhaust system for the automotive vehicle, for example, the catalyst substrate and shock absorbent member are applied with force in their axial directions, due to vibration or exhaust gas pressure. In opposition to the axial force, a holding force is required for them in the axial (longitudinal) direction of the cylindrical housing, which holding force is created by first frictional force between the shock absorbent member and the catalyst substrate, and second frictional force between the shock absorbent member and the cylindrical housing.
The first and second frictional forces are indicated by the product of multiplying the pressure reproduction force of the shock absorbent member and the static coefficient of friction between the shock absorbent member and the outer surface of the catalyst substrate, and the product of multiplying the pressure reproduction force of the shock absorbent member and the static coefficient of friction between the shock absorbent member and the inner surface of the cylindrical housing, respectively. In this respect, as for the holding force in the axial (longitudinal) direction of the cylindrical housing, the frictional force between the shock absorbent member and the remaining one with the smaller coefficient of friction is dominant. With respect to the catalyst substrate and cylindrical housing with known static coefficients of friction, therefore, frictional forces are made clear. In order to ensure the requisite frictional forces, it is required to increase the pressure applied to the shock absorbent member. In the case where the catalyst substrate is fragile, it is required to ensure the axial holding force within the pressure limit to the shock absorbent member, to avoid excessive radial load applied to the catalyst substrate.
Accordingly, it is preferable to determine the pressure applied to the shock absorbent member, on the basis of the one with the smaller static coefficient of friction, out of the static coefficient of friction of the outer surface of the catalyst substrate and the static coefficient of friction of the inner surface of the cylindrical housing, and reduce the diameter of the cylindrical housing. In other words, when holding the catalyst substrate in the cylindrical housing with the shock absorbent member disposed between them, most appropriate parameter is the pressure (Pascal) applied to the catalyst substrate (or, filter) through the shock absorbent member (shock absorbent mat). If it is possible to measure the pressure directly, or measure a value directly corresponding to, or similar to, the pressure, and reduce the diameter of the cylindrical housing on the basis of one of the measured results, then it is possible to reduce the diameter of the cylindrical housing by a sizing process, with satisfactory accuracy. The sizing process means reducing the diameter of the cylindrical housing, controlling the reduced amount, and is distinguished from mere shrinking process for simply reducing the diameter of pipe, which may fall within the same category as that in the sizing process in terms of the process for reducing the diameter of the cylindrical housing.
On the contrary, in the prior methods, generally employed is a control on the basis of the GBD of shock absorbent member (mat) as described before, so that a control through an estimation on the basis of a substituted value has been employed. Therefore, those estimated factors are added together to cause the unavoidable error. Also, the holding force that is caused by the frictional force between the shock absorbent member and catalyst substrate, and the holding force that is caused by the frictional force between the shock absorbent member and cylindrical housing, are eventually confused with each other, to determine the dimensions of each parts. In the measurement as described in the Japanese Patent Laid-open Publication No. 2001-355438 as described before, the estimated factors for the following processes are necessarily added together to cause the error, against which countermeasures will be required.
Especially, in the filed of the catalytic converter, it is desirable that the pressure of the shock absorbent mat is made as strong as possible, and applied uniformly in the peripheral and axial directions, in view of the variation or aged change in pressure resulted from the error in the outer diameter of the catalyst substrate, or the pressure (whose minimum pressure is indicated by a) for preventing the catalyst substrate from moving in the axial direction of the catalyst substrate due to various accelerations when in use. If the compression force is provided to be excessive so as to satisfy the desire as described above, the catalyst substrate might be fractured, so that the pressure can not be made greater than a predetermined pressure. The pressure that is applied when the catalyst substrate is fractured, is called as isostatic strength xcex2. Furthermore, in response to recent requirement of further improvement in exhaust purifying performance, further reduction in wall thickness has been required, so that the catalyst substrate is getting much more fragile than the prior catalyst substrates, i.e., large reduction in xcex2, a range for allowing the holding force to be set, which can be indicated by a fracture margin to the pressure (xcex2xe2x88x92xcex1), will be further narrowed.
Furthermore, increase in temperature of the exhaust gas (temperature of the gas fed into the catalytic converter) will be caused to reach approximately 900 degrees centigrade, so that it is required to combine the shock absorbent mat with alumina mat having a high temperature resistance. However, as the alumina mat does not have thermal expansion property, it is difficult to conform the alumina mat to a change in shape of the metallic container having thermal expansion property. In view of this, the minimum pressure xcex1 is required to be set larger than that set for the conventional process, and the bulk density of the shock absorbent mat is required to be set relatively large. Recently, therefore, it is likely that the reduction in xcex2 and increase in xcex1 will result in a large reduction in the pressure allowance range (xcex2xe2x88x92xcex1), which will be described later in detail with reference to FIG. 28. In other words, accurate determination of the pressure is requisite for each product, to result in difficulty in mass-production of the catalytic converter. In addition, recent progress in narrowing the wall thickness of the catalyst substrate for use in the catalytic converter causes the pressure allowance range (xcex2xe2x88x92xcex1) to be approximately half of the prior range, and it is estimated that further narrowing the wall thickness will cause it to be approximately half of the present range. As can be seen from those narrow ranges, it is apparent that it will be very difficult to fit such thin wall catalyst substrate into the cylindrical housing by means of the prior stuffing process or the like, maintaining the appropriate pressure to be applied.
Accordingly, it is an object of the present invention to provide a method and an apparatus of producing a container for holding a columnar member in a cylindrical housing, with a shock absorbent member wrapped around the columnar member, to achieve an appropriate sizing process to the cylindrical housing on the basis of the pressure applied to the columnar member by a compression reproduction force of the compressed shock absorbent member, thereby to hold the columnar member with the shock absorbent member wrapped around it in the cylindrical housing, appropriately.
And, it is another object of the present invention to provide a method and an apparatus capable of producing a container for holding a columnar member in a cylindrical housing, properly adjusted to a change in wall thickness and spring back of the cylindrical housing, which are resulted from reducing a diameter of the cylindrical housing when sizing it.
In accomplishing the above and other objects, the method comprises the steps of (1) compressing at least a part of the shock absorbent member wrapped around the columnar member, by a pushing member in a radial direction toward a longitudinal axis of the columnar member, (2) measuring a pressure applied to the shock absorbent member by the pushing member, (3) measuring a distance between the axis of the columnar member and an end of the pushing member contacting the shock absorbent member, when the measured pressure substantially equals a predetermined target pressure, to provide a target radius, (4) inserting the columnar member with the shock absorbent member wrapped around the columnar member, into the cylindrical housing loosely, and (5) reducing a diameter of at least a part of the cylindrical housing with the shock absorbent member held therein along the longitudinal axis of the cylindrical housing, with the shock absorbent member being compressed, to such an extent that the inner radius of the part of the cylindrical housing substantially equals the target radius, to hold the columnar member with the shock absorbent member wrapped around the columnar member and compressed at the target pressure, in the cylindrical housing.
In the method as described above, preferably, the target pressure is determined on the basis of a static coefficient of friction of the outer surface of the columnar member, and a static coefficient of friction of the inner surface of the cylindrical housing, and pushing force of the pushing member applied to the shock absorbent member.
In the method as described above, a plurality of pushing members may be placed around the periphery of the columnar member in parallel with the longitudinal axis thereof, and at least one of the pushing members may compress the shock absorbent member wrapped around the columnar member in the radial direction toward the longitudinal axis of the columnar member, to measure the pressure applied to the shock absorbent member.
Preferably, the plurality of pushing members comprise a plurality of elongated members, each having a length corresponding to the part of the cylindrical housing with the shock absorbent member held therein, and wherein the plurality of elongated members are placed in parallel with one another around the periphery of the shock absorbent member wrapped around the columnar member.
In the method as described above, preferably, a predetermined amount of correction is provided on the basis of at least one of a change in diameter and a change in thickness of the cylindrical housing when the diameter of the cylindrical housing is reduced, and the reducing amount of the cylindrical housing is adjusted according to the amount of correction, when the diameter of the cylindrical housing with the shock absorbent member held therein is reduced.
And, the amount of correction may be provided by measuring a limit radius of the cylindrical housing, when the shock absorbent member is compressed by the pushing member to such an extent that the inner radius of at least the part of the cylindrical housing is reduced to be less than the target radius and immediately before the columnar member will be fractured, and setting a predetermined distance less than a difference between the limit radius and the target radius, as the amount of correction.
As for the apparatus of producing a container for holding a columnar member in a cylindrical housing with a shock absorbent member wrapped around the columnar member, it includes a compression device having a plurality of elongated pushing members, each having a length corresponding to at least a part of the cylindrical housing with the shock absorbent member held therein, and being placed in parallel with one another around the periphery of the shock absorbent member wrapped around the columnar member, and compressing at least the part of the shock absorbent member wrapped around the columnar member, by the pushing members in a radial direction toward a longitudinal axis of the columnar member. The apparatus further includes a measuring device for measuring a pressure applied to the shock absorbent member by the pushing members, and measuring a distance between the axis the columnar member and an end of at least one of the pushing members contacting the shock absorbent member, when the measured pressure substantially equals a predetermined target pressure, to provide a target radius, and a control device for inserting the columnar member with the shock absorbent member wrapped around the columnar member into the cylindrical housing loosely, and driving the compression device to reduce a diameter of at least the part of the cylindrical housing with the shock absorbent member held therein along the longitudinal axis of the cylindrical housing, by the pushing members, to such an extent that the inner radius of the part of the cylindrical housing substantially equals the target radius, to hold the columnar member with the shock absorbent member wrapped around the columnar member and compressed at the target pressure in the cylindrical housing.
As an embodiment of the columnar member container, a catalytic converter for use in an automotive vehicle is produced. Or, a diesel particulate filter (DPF) may be produced. With respect to the catalytic converter, the columnar member corresponds to a catalyst substrate, e.g., the substrate of a honeycomb structure, and the shock absorbent member corresponds to a shock absorbent mat for holding the substrate. With respect to the DPF, the columnar member corresponds to a filter, and the shock absorbent member corresponds to a shock absorbent mat for holding the filter. In general, the substrate or filter corresponding to the columnar member is formed into a column with a circular cross section or a cylinder. According to the present invention, however, the columnar member includes the one with a noncircular cross section, such as elliptic cross section, oval cross section or the like. In this case, a half of the mean value of its major axis and minor axis may be served as the radius of the cylindrical housing according to the present invention.